Soft shape memory in main-chain liquid crystalline elastomers

Abstract

The field of shape memory polymers (SMPs) has been dominated by polymeric network systems whose fixing mechanism is based on crystallization or vitrification of the constituent chains, rendering such systems stiff in comparison to elastomers, gels, and living tissues. In this report, we describe the synthesis and characterization of liquid crystalline elastomers that exhibit both bulk and surface shape memory effects with compositionally dependent transition temperatures that determine the shape fixing and shape recovery critical temperatures. Main-chain, segmented liquid crystalline elastomers were synthesized using hydrosilylation linking of poly(dimethylsiloxane) oligomers with mesogenic dienes of two compositions and a tetrafunctional crosslinker. Calorimetric and dynamic mechanical analyses revealed two composition-determined thermal transitions for the LCEs, including a glass transition event at low temperatures (33 °C < T_g < 48 °C) and a first-order isotropization transition at higher temperatures (57 °C < T_i < 71 °C), each increasing with an increase in the concentration of the more slender, unsubstituted mesogenic diene, 5H. Despite the existence of a glass transition event, the materials remain soft at low temperature, a finding explained by vitrification of only the mesogen-rich layers within the smectic phase. Shape memory behavior was evaluated quantitatively and revealed excellent shape fixing and shape recovery values, generally in excess of 98%, with the recovery temperature depending on composition in a manner determined by the LCE phase behavior, particularly T_g. The temperature-dependent kinetics of shape memory were analyzed for a selected LCE composition, revealing exponential time dependence with rate constants that depended on temperature in an Arrhenius manner. Finally, the softness of the LCE SMPs was exploited to fix an embossed, micron-scale pattern on the surface and then recover the equilibrium flat state quite completely. We envision application of this surface shape memory phenomenon in the areas of soft lithography, especially microcontact printing and microfluidics.